1. Field of the Invention
This invention relates to dimensionally recoverable articles, particularly heat-recoverable articles, and to compositions suitable for making such articles.
2. Introduction to the Invention
Dimensionally recoverable articles such as heat-recoverable tubing are well-known. Such articles are often used to provide electrical and environmental insulation over spliced wires or to protect areas of cables where there are breaks in the insulation. To provide adequate recoverability and flexibility, the articles generally comprise polymers. For many applications it is necessary that the polymer be flame-retarded in order to minimize the risk of damage in the event of a fire. Such flame-retardancy may be achieved either by the use of a halogen-containing polymer, e.g. a fluoropolymer, or by the addition of a halogenated material, e.g. a brominated or chlorinated compound such as decabromodiphenylether (DBDPE), to a non-halogenated polymer, e.g. polyolefins such as polyethylene or ethylene copolymers. Polymeric compounds with such flame retardancy are useful as they self-extinguish when removed from flame. However, they do have disadvantages, as they can undergo continuous combustion to produce a large quantity of smoke, or they can thermally decompose, releasing corrosive gases, e.g. halogen acids, which can be detrimental to personnel or sensitive electronic equipment.
Attempts have been made to incorporate halogen-free flame-retardants into polyolefin resins to overcome the above disadvantages. Halogen-free flame-retardants such as alumina trihydrate (ATH) must be used in relatively large concentration in order to give fully useful flame-retardancy to the resulting compositions. However, when a large concentration of halogen-free flame-retardant is added to a polyolefin resin, the resulting resin composition, when extruded, exhibits reduced physical properties, especially in tensile strength and elongation, and also greatly reduced aging and electrical properties. In addition, these compositions are difficult to extrude and expand into tubing, especially thin wall tubing.
When the composition is used in the form of tubing, a number of product-related flame tests are conducted. Of particular importance is the Underwriters' Laboratories UL Standard for Safety for Extruded Insulating Tubing, UL 224, Fifth Edition, Dec. 15, 1999 (referred to hereinafter as the Fifth Edition VW-1 Flame Test), the disclosure of which is incorporated herein by reference. UL 224 contains several different aspects, including measurements of the physical properties, i.e. tensile strength and elongation, and a measurement of the performance of the tubing when exposed to a flame, i.e. the VW-1 (Vertical Wire) flame test. In this test, the tubing is recovered and is then supported by steel music wire while five applications of flame are made. To pass the test, the tubing must not flame or glow more than sixty seconds following any of the five flame applications.
There are a number of issues associated with the VW-1 flame test. First, the lack of a heat sink, as provided by a metal mandrel in other tests, means that the polymer gets hotter, increasing the chances of flaming and making the test difficult to pass. Second, under the provisions of the Fifth Edition test, the UL chamber used for testing has been redesigned; it is much larger and is completely sealed, limiting any drafts in the chamber and making the test more difficult to pass than in older versions of the test. In the test version using the former chamber, provisions were made to allow a draft to be present as long as there was no effect on the character of the applied flame. The draft served to remove any smoke that was generated during the test, but also assisted in damping some types of sputtering flames that might be generated on the polymer. In the Fifth Edition test, the sealed chamber ensures that any sputtering flame on the polymer will not be extinguished. Therefore, products that may have passed the VW-1 test using the older chamber now may have difficulties in passing. Third, it is desirable that tubing made from the same composition should pass the test for a variety of different wall thicknesses and inner diameter values. However, while very large tubing (i.e. tubing having an inner diameter greater than 10 mm) usually passes, as the volume of polymer present makes it difficult to heat sufficiently to flame or glow, and very small tubing (i.e. tubing having an inner diameter less than 2 mm) usually passes, as it is overwhelmed by the heat and burns quickly, intermediate size tubing, i.e. tubing with an inner diameter of 3 to 9 mm often does not pass the test. This is due to the construction of these intermediate sizes, the wall thickness, and the concentration of the flame retardant in the polymer wall.
It is also desirable that the tubing have adequate performance for continuous use at elevated temperature, i.e. at 125° C. Such high temperature performance is particularly important when the tubing is used for automotive applications.
Non-halogen-containing tubing made from a polyethylene-based polymeric composition in which the flame retardant comprises a mixture of a hydrated or carbonated inorganic filler and red amorphous phosphorus is known. While such tubing may have acceptable physical properties, not all sizes of such tubing are able to pass the Fifth Edition VW-1 test, making it unsuitable for applications, including automotive applications, that require passage of such a test. It is therefore, desirable to have a composition for use in heat-recoverable articles such as tubing that exhibits good high temperature performance, has good physical properties, and has the ability to pass the Fifth Edition VW-1 test across a range of sizes. An example of conventional non-halogenated tubing is MTZH™ tubing, available from Tyco Electronics Corporation.